Cryptosporidium is an obligate, intracellular coccidian protozoa that replicates in small-bowl epithelial cells of a vertebrate host. The main mode of transmission is through contact with contaminated water. Occasionally it can be transmitted through contaminated food, but it is mostly through contaminated water. Cryptosporidium eggs come in two varieties, thick-walled and thin-walled. Thin-walled eggs are the ones involved in autoinfection, while thick-walled eggs are passed from the infected host via stool. Thick-walled eggs are immediately infective and can contaminate water or transmit directly via a fecal-oral route. The risk of infection is so high because very few eggs are required to infect a new host. The leading cause of Cryptosporidiosis is swallowing infected water, frequently from pools or water playgrounds. The usual levels of chlorine in pools and drinking water is not enough to kill the thick-walled eggs, and freezing does not disrupt them either. Even though it only takes a few eggs to make someone sick (>100), an infected person can excrete 10 to 100 MILLION eggs.
Symptoms usually show up about 7 days after infection, and they may become apparent abruptly. The most prominent symptom is abdominal cramps and profuse, watery diarrhea. However, nausea, vomiting, fever, weakness, and loss of appetite can also affect an infected person. These symptoms usually last for 1 to 2 weeks, however eggs may be excreted for weeks after symptoms have resolved.